The term cryptovolcanic structure has been used for complexes of approximately circular cross-section, superficially like volcanic pipes (necks) but containing little or no volcanic material. Limiting dimensions are not normally included in the definition but most of the bodies for which the term has been used are at least 2 km in diameter. The rocks in similar but smaller bodies, circular pipes, or bodies with irregular cross-section (even dyke-like) have been called intrusivetuffs. The large masses show signs of central uplift consisting of broken-up and contorted strata surrounded by a ring-shaped depression (rim syncline) that may be bounded or cut by faults. The whole structure often makes a topographical basin. Frequently a zonal form is recognized by the type and distribution of the fragmented material; marginal zones are made up of local wall rock, but central parts commonly consist of blocks of exotic material, mostly derived from deeper levels. The matrix typically consists...
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